In gantt v.4.3.7, we are trying to add a subask with duration of 1 day and start date matching the parent. I've tried the following based on this post: viewtopic.php?p=94027#p94027
Where did you put that piece of code inside your configuration?? First of all, we recommend updating to the latest version of the Bryntum suite, also, are you able to reproduce that behavior in our demos?? We need more information about how is the component set up to assist you better.
Essentially, we are trying to add a subtask with custom values. If it gets added with the default values, it messes up some logic we have tied to the Parent task.